Fact check: Available mRNA vaccines do not target syncytin-1, a protein vital to successful pregnancies

Numerous posts have falsely claimed that mRNA vaccines used against COVID-19 target a protein called syncytin-1, which is needed for placental formation and successful pregnancies. Many of these posts also baselessly imply that the vaccines will make people infertile. These claims are untrue; no available mRNA vaccines target a protein called syncytin-1.

Reuters Fact Check. REUTERS

One such post (here) reads: “The mRNA vacks targets a protein called Syncitin-1 which is a protein contained in the CV spike protein complex.

“The PROBLEM is, there is no mechanism to turn off or STOP the destruction of Syncitin-1 which is also found in the BRAIN and female reproductive system. Therefore, Syncitin-1 will continue to be scavenged in the body. Without Syncitin-1 no placenta can form. The brain manifestations of no Syncitin-1 are MS, Parkinsons, Schizophrenia, Psychopathy and I posit; any neurodegenerative condition.” [sic]

The Pfizer/BioNTech and Moderna COVID-19 vaccines work by inserting mRNA which tells human cells to produce a sequence of amino acids that forms a protein like the “spike protein” found on the SARS-CoV-2 virus which causes COVID-19. The mRNA is then destroyed (here).

The aim is for the immune system to recognise and eliminate this protein so that, if it encounters an intact SARS-CoV-2, it is prepared to target the same structure on the virus.

The claims that these mRNA vaccines will cause the immune system to attack syncytin-1 are unfounded. Syncytin-1 is not contained in the SARS-CoV-2 spike protein and the SARS-CoV-2 spike protein and syncytin-1 are not very similar.

Pfizer spokesperson Dervila Keane confirmed to Reuters by email previous reports (here) that the protein targeted by their vaccine only shared a sequence of four amino acids with syncytin-1, which is too short to cause autoimmunity problems. Syncytin-1 is made up of 538 amino acids here).

“Given that there are only 20 different types of amino acid, it isn’t surprising that many, many proteins share similarities”, wrote Catherine Thornton, a Professor of Human Immunology at Swansea University (here) and April Rees, a PhD Researcher in Immunology at Swansea University in an article on the topic for The Conversation publication here .

Numerous experts have confirmed that there are few similarities between the COVID-19 spike protein and syncytin-1 and that these were not significant enough to for the immune system to be confused between the two (here and here and here and here and here).

Some pointed out that the same immune response triggered by the mRNA vaccine response would be seen in pregnant women who had contracted COVID-19 and there was no evidence of increased miscarriages hospitalisations or placental abnormalities in this group (here and here and here).


False. Syncytin-1 is not part of the SARS-CoV-2 spike protein. There is no evidence that mRNA vaccines for COVID-19 have a negative effect on female fertility.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts  here  .